unpack_bootimg: fix pylint errors am: e71c440ea1 am: a17e34353f am: 884f5bb06e am: 0f5ad04242

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2044946

Change-Id: I759dcacc1a4a9fb80738de85cea8ad2f2e3a9cfa
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>